Magnetic field structure for transducers and method of constructing same



Feb. 26; 1963 A A. SARIT 3,079,472

. l MAGNETIC FIELD STRUCTURE FOR TRANSDUCERS AND METHOD OF CONSTRUCTING SAME Filed Oct. 6, 1960 INVENTOR. Anthem; A. Sarih mo mwq Sttes This invention relates to magnetic field structures for loudspeakers or the like, and more particularly to field structures employing ferrite magnets and to means for holding the parts of such field structures together.

In loudspeakers which utilize ferrite magnets as part of their field structures, difficulty has been encountered in securing the front and back plates to the ferrite magnets without interfering with the flow of flux between the ferrite magnet and the plates. One method of holdingthe structure together is to place a plastic coating around the outer surface of the whole assembly, but this has been found to be very expensive and time consuming. Another known method of securing the magnetic plates to the ferrite magnets is to use a fastening material such as cement. However, this causes gaps to be formed between the plates, thereby increasing the reluctance between the plates and the magnet.

It is therefore a primary object of this invention to provide an improved magnetic field structure which is free from the aforementioned difficulty.

It is another object of this invention to provide an improved magnetic field structure for loudspeakers or other electromagnetic transducers which will allow a maximum and uniform flow of flux through the field structure.

It is still another object, of this invention to provide an improved method of constructing a field structure as aforesaid.

It is also an object of this invention to provide an improved magnetic field structure which is of rugged construction and which will withstand severe shock without affecting the form of the unit.

It is still a further object of this invention to provide an improved magnetic field structure as aforesaid which is simple and economical to manufacture and which is efficient to use.

According to one form of this invention, a magnetic field structure for a loudspeaker or the like is provided which contains a front and a back magnetic plate, each of which has a central, circular opening therethrough. Formed in the surfaces of the magnetic plates around the peripheries of the respective openings are a plurality of slots. One end of a circular, magnetic pole piece is press-fitted within the circular opening in the back plate. A suitable magnet, preferably of a ferrite and with a circular opening therethrough, is placed about the pole piece and rests on the inner surface of the back plate. A non-ferrous, tubular holder member, which has protrusions extending outwardly from the opposite ends thereof, is positioned around the pole piece with one set of the protrusions extending into the slots in the back plate. The front plate is placed on top of the magnet so that the other set of protrusions extending outwardly from the holder member is positioned in the slots in the front plate. The end of the pole piece which extends into the opening in the front plate has a diameter which is less than the diameter of the opening. The space between the pole piece and the front plate serves as an air gap in which a coil for a loudspeaker or other type of transducer can be positioned. The two plates are clamped and held firmly against the magnet by applying pressure, to the ends, of the protrusions, thereby deformice ing them laterally tightly against the front and back plates. The air gap is maintained during the deforming process by inserting a centering gauge in the space between the pole piece and the front plate. If the field structure is to be used in a loudspeaker, the speaker housing is affixed to the outer surface of the front plate in any suitable manner. The voice coil is positioned in the air gap as heretofore described.

The novel features of this invention, both as to its organization and method of assembly, as well as to additional objects and advantages thereof, will be understood rnore fully from the following description when read in connection with the accompanying drawing, wherein:

FIGURE 1 is an exploded, perspective view of the various elements of the field structure according to one form of the invention;

FIGURE 2 is a plan view, partly in section taken along the line 2-2 of FIGURE 3, of the magnetic field structure of FIGURE 1 as assembled; and

FlGURE 3 is a sectional view of a complete loudspeaker taken along the line 33 of FIGURE 2.

Referring, now, more particularly to FIG. 1 of the drawing, a magnetic field structure 10 is shown having a front magnetic plate 12 and a back magnetic plate 14. The back plate 14 is formed with a central, circular opening 16 and with a plurality of recesses in the form of slots 18 therethrough, the slots 18 being disposed around the periphery of the opening 16 and extending radially outwardly therefrom. The slots 18 are adapted to receive protrusions extending from thesurface of a holder to be hereinafter described. The front plate 12 has a circular opening 21) and slots 22 therethrough which are similar to and axially aligned with those in the back plate 14. Both of the plates 12 and 14 are preferably formed of low carbon sheet in order to improve their permeability, although other types of ferrous material could be used.

A magnetic pole piece 24 having a base portion 26, and a neck portion 28 which is slightly smaller in diameter than the base portion 26 is affixed to the back piate 14. The bottom edge of the pole piece 24 is chamfered inwardly so as to be easily pressed in the opening 16 in the back plate 14. The base portion 26 is of substantially the same diameter as the opening 16 in the back plate 14 so that it can be secured within the back plate 14 by press-fitting it in the opening 16. The neck portion 28 of the pole piece 24 has a smaller diameter than the opening an in the front plate 12, so that when the neck portion 28 is positioned within the opening 16, an air gap 30 is formed therebetween in which a voice coil to be hereinafter described can be inserted.

Magnetic flux is provided by a ferrite magnet 33 which has a circular opening 34 therein of greater diameter than the outside diameter of a tubular holder member 32 to be hereinafter described more particularly. The magnet 33 is formed in substantially the same shape as the front and back plates 12 and 14 which can be rectangular, circular or of any other desired shape.

The holder member 32 has a hollow, cylindrical, center section 43 which has a bore 42 therein and a plurality of circumferentially spaced protrusions 44 extending from opposed ends of the central body section 4t). The protrusions 44 are adapted to fit into the slots 18 and 22 in the front and back plates 12 and 14 and to be staked against these plates as hereinafter more fully described. The holder member 32 is formed of a non-ferrous material such as aluminum or powdered brass or of any other type of material which will not appreciably affect the flow of flux in the field structure 10 The front, plate 12 is also formed with a. number of;

protrusions 46 extending outwardly from the upper surface 47 thereof. These protrusions can be force fit into apertures in a housing 48 to which the field structure is to be connected. If the magnetic field structure 10 is to be used in a loudspeaker, as illustrated in FIG. 3, a voice coil 50 which is mounted on the voice coil form of a diaphragm 52 is positioned within the air gap 30. It will be understood, however, that the magnetic field structure 10 can be utilized with other types of devices, such as microphones, etc.

In order to assemble the magnetic field structure 10, the following steps are followed: The base portion 26 of the pole piece 24 is press-fitted within the circular opening 16 in the back plate 14 with the chamfered edge extending out past the outer surface 54 of the back plate 14. The holder member 32 is inserted over the pole piece 24 with the protrusions 44 projecting from one end of the central portion 40 extending into the slots 18. The protrusions 44 extend a short distance outwardly past the outer surface 54 of the back plate 14. The center portion 40 of the holder member 32 rests against the inner surface 56 of the back plate 14. The ferrite magnet 33 is then placed on the inner surface 56 of the back plate 12 about the neck portion 28 of the pole piece 24, so that the neck portion 28 will extend through the circular opening 34 in the magnet 33. The front plate 12 is then placed on the top surface of the ferrite magnet 33, the upper protrusions 44 of the holder member 32 being received in, and extending through, the slots 22 in the front plate 12. The neck portion 28 of the pole piece 24 is positioned within the opening in the front plate 12. The air gap 30 is formed between the pole piece 24 and the wall of the opening 20 in the front plate 12 inasmuch as the diameter of the pole piece 24 is less than the diameter of the opening 20. Next, a centering gauge (not shown) is inserted in the air gap 30. Pressure is then applied to the ends of the protrusions 44, by any suitable device, in order to bend the protrusions 44 away from the axis of the holder member 32 and tightly against the outer surfaces of the plates 12 and 14. The deformed protrusions 44, thus staked against the plates 12 and 14, secure the plates 12 and 14 to the end surfaces of the ferrite magnet 33 by a clamping action. With the plates 12 and 14 secured against the ferrite magnet 33, the holder member 32, which is positioned between the plates, acts as a dust shield between the ferrite magnet 33 and the air gap 3% thereby preventing any particles of magnetic dust from moving into the air gap 30 and thus hindering the movement of the voice coil 50 within the air gap. The protrusions 44 will not interrupt the flow of flux between the front plate 12 and pole piece 24- except in the small areas where the protrusions 44 are located. The centering gauge is then removed. Next, the cone housing 48 is secured to the surface of the front plate 12 by force or press fitting the protrusions 46 within correspondingly located apertures in the housing 48. The voice coil of the speaker extends to the air gap 36.

From the foregoing description, it will be apparent that I have provided an improved field structure for loudspeakers or the like, as well as an improved method of holding together the elements of the magnetic field structure. This structure and method are particularly useful in the case of field structures employing ferrite magnets. While I have illustrated and described one particular type of magnetic field structure, it will, no doubt, be apparent to those skilled in the art that various changes may be made in the particular form disclosed, and that other forms are possible, all within the spirit of the present invention. Hence, I desire that the foregoing be taken merely as illustrative and not in a limiting sense.

What is claimed is:

A magnetic field structure for an electromagnetic transducer comprising a pair of spaced magnetic plates each having an Opening therein, a magnetic pole piece secured within one of said openings and extending into the other of said openings in radially spaced relation to the periphery of said other opening to provide an air gap, magnet means disposed between and in engagement with said plates, and a clamping member extending between said plates and through both said openings for clamping said plates against said magnet means to provide firm engagement therebetween.

2. A magnetic field structure for an electromagnetic transducer comprising a pair of spaced magnetic plates each having an opening therein, a magnetic pole piece secured within one of said openings and extending into the other of said openings in radially spaced relation to the walls of said other opening to provide an air gap, magnet means disposed between said plates about said pole piece, said magnet means engaging said plates, and a cylindrical clamping member extending between said plates and through both said openings for clamping said plates against said magnet means to provide firm engagement therebetween.

3. A magnetic field structure for an electromagnetic transducer comprising a pair of spaced magnetic plates each having an opening therein, a magnetic pole piece secured within one of said openings and extending into the other of said openings in radially spaced relation to the walls of said other opening to provide an air gap, magnet means disposed between said plates about said pole piece, said magnet means engaging said plates, and a cylindrical clamping member for clamping said plates against said magnet means to provide firm engagement therebetween, said cylindrical clamping member being disposed around said pole piece between said pole piece and said magnet, said clamping member also extending between said plates and through both said openings.

4. A magnetic field structure for a transducer comprising a pair of magnetic plates each having an opening therethrough, a magnetic pole piece secured within one of said openings and extending into the other of said openings, a magnet positioned between said plates, and a holder member positioned adjacent said pole piece, said holder member extending through both said openings and being staked to said plates to thereby secure said plates to the surfaces of said magnet.

5. A magnetic field structure according to claim 4 wherein said holding member is made of non-ferrous material.

6. A magnetic field structure for a transducer comprising a pair of magnetic plates each having an opening therethrough, said plates each having recesses in the peripheries of their respective openings, a magnetic pole piece press-it within one of said openings and extending into the other of said openings in radially spaced relation to the periphery of said other opening to define an air gap, a magnet positioned between said plates, and a holder member securing said plates to said magnet, said holder member extending into said recesses and being staked to each of said plates adjacent said openings thereof.

7. A magnetic field structure for a transducer comprising a pair of magnetic plates each having an opening therethrough, and a plurality of slots disposed around the periphery of said openings, a magnetic pole piece pressfit within one of said openings and extending into the other of said openings in radially spaced relation to said other opening to define an air gap, a magnet positioned between said plates, and a holding member having a plurality of protrusions extending from opposite ends thereof, said protrusions on one of said opposite ends extending into said slots in one of said plates, said protrusions on the other of said opposite ends of said holder member extending into the slots in the other of said plates, said protrusIons being deformed laterally beyond said slots and being staked tightly against said plates to firmly secure said plates to the surfaces of said magnet to thereby retain said magnet against lateral movement relative to said plates.

8. A magnetic structure for an electroacoustical transducer comprising a pair of magnetic plates each having a circular opening therethrough, said plates having in the peripheries of their said openings a plurality of slots, a circular pole piece press-fit within one of said openings and centered Within the other of said openings in spaced relation thereto to provide an air gap, a magnet positioned between said plates and around said pole piece, and a hollow, cylindrical holder member having a plurality of protrusions extending from the opposite ends thereof, one set of said protrusions being positioned Within said slots in one of said plates and the other set of said protrusions being positioned in the slots of other of said plates, said protrusions being deformed laterally beyond said slots and being staked tightly against the outer surface of each of said plates to thereby firmly attach said plates to the surfaces of said magnet whereby to retain said magnet against lateral movement relative to said plates.

References Cited in the file of this patent UNITED STATES PATENTS 2,197,649 Neuschatz Apr. 16, 1940 2,312,233 Cunningham Feb. 23, 1943 2,341,539 Giannini Feb. 15, 1944 10 2,486,837 Goldsmith NOV. 1, 1949 FOREIGN PATENTS 959,833 Germany Mar. 14, 1957 

1. A MAGNETIC FIELD STRUCTURE FOR AN ELECTROMAGNETIC TRANSDUCER COMPRISING A PAIR OF SPACED MAGNETIC PLATES EACH HAVING AN OPENING THEREIN, A MAGNETIC POLE PIECE SECURED WITHIN ONE OF SAID OPENINGS AND EXTENDING INTO THE OTHER OF SAID OPENINGS IN RADIALLY SPACED RELATION TO THE PERIPHERY OF SAID OPENING TO PROVIDE AN AIR GAP, MAGNETIC MEANS DISPOSED BETWEEN AND IN ENGAGEMENT WITH SAID PLATES, AND A CLAMPING MEMBER EXTENDING BETWEEN 